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The late May skies were grey above the Idaho road, its blacktop patchy with ice, when Leah Woodward and her husband watched two cars collide head-on.

The screams became audible, Woodward said, when she and her husband ran from their car to help pull survivors from the crash.

"We could hear a kid screaming, a little baby screaming," Woodward told Idaho TV station KBOI. Her husband, using a trailer hitch as a club, split open the back window of the car that held the wailing passenger.

Inside was a young boy strapped to a booster seat, Woodward said, surrounded by a halo of pinkish fluid. Later, as she wrote on Facebook, she would find out that the liquid had leaked from a wound to his spine.

Woodward's husband, a Nampa, Idaho, police officer with first responder training, helped his wife stabilise the boy's neck. As her husband consoled Brandy Gonzalez - the driver and boy's mother - Woodward held the boy's head as still as possible.

Using a blanket she swaddled the child to keep Gonzalez, who was close to shock from her own injuries, from seeing how badly the accident had hurt her son, named Killian.

"I'm trying to stay calm but inside I'm panicking," Woodward said in the KBOI interview. "I'm thinking I don't know what I'm doing, and it was the worst feeling I've ever had to not know how to help."

For more than half an hour, Woodward cradled Killian, rigidly supporting his neck until paramedics arrived, and flew the child to a hospital via helicopter ambulance.

Gonzalez and her son had been driving to Idaho after celebrating Killian's fourth birthday in Nevada. A passing hail storm left the road icy and slick, and when her car hit the ice it skidded out of control and into another SUV. That driver was not seriously harmed. Gonzalez fractured bones in her left and right legs, as well as her left arm.

And though her injuries were bad, Killian's were worse.

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His spleen was ruptured, his arms and multiple ribs broken. But the most alarming trauma was to his neck. What Killian suffered goes by several clinical names, including atlanto-occipital dislocation, or, more evocatively, internal decapitation.

Tough ligaments connect the human skull to the top of the spinal column. During a so-called internal decapitation, those ligaments stretch or snap, and the skull separates from the supporting spinal bones. From the outside, however, the neck appears intact.

The odds of living through an internal decapitation are low, with some studies reporting less than one survivor out of every 10 such injuries. In one retrospective report of 69 cases of internal decapitation, 47 people died instantly and 15 perished at a hospital. Only seven of the 69 lived to be discharged from a hospital.

"It is not an uncommon injury for people who arrive DOA [dead on arrival]," University of Kentucky neurosurgeon Phillip Tibbs told ABC News in 2008. "It is rare to survive this."

The severity of the injury hinges on damage to the spinal cord - the nerve fibre superhighway that connects the brain to the rest of the body, running through the spinal column. In many cases of internal decapitation, the impact also fatally tears the spine.

Children are at higher risk for the injury - due to the fact their heads are proportionally larger in comparison to their bodies, and because their ligaments are not as strong. But they seem to survive at greater rates than adults, even in some unusually dire circumstances.

In 2008, for instance, a 9-year-old boy lived through a car accident that doctors shoved his head an inch forward, separating it from his spine while he had been fastened into the car with a seat belt.

After the initial impact, however, internal decapitation can still leave children paralysed or neurologically impaired. To stop "catastrophic neurologic injury," as one group of spinal experts put it, it is crucial to keep the head and neck stabilised in a line.

Woodward's actions as a Good Samaritan are being hailed as the difference between tragedy and Killian's survival. In fact, reports KTLA 5 News, the boy did not need to undergo surgery for his neck.

"He's doing amazing. He's shocked everyone there. They keep telling me he's the talk of the hospital," said Gonzalez, who was recovering at a separate hospital and had not, as of last weekend, been reunited with her son.

"My heart melted into that little boy as I held him," Woodward wrote on Facebook, "and it'll always be there."

To defray the cost of their medical bills, Gonzalez has created a GoFundMe page for her and her son.